Networks are known in which the primary transmission medium is a cable characterized by relatively high-transmission rates. Network nodes connected directly to the cable medium may typically consist of shared network resources, such as host computers, mass storage media, and communications ports. To better control data flow and allow a greater number of network users, the network may be configured as several local area networks (LAN's). Each LAN will comprise a limited number of devices that communicate directly with one another over local cables. The LAN devices may be coupled to the primary cable medium by a communication controller which is normally a router or a bridge. Although a router and a bridge operate in different ways, both ultimately enable data to be transferred between a device in a particular LAN and a device connected directly to the primary cable medium or located in another LAN.
The operation of a conventional router is well known. It will typically create and maintain a table of LAN's within the network. It is effectively unaware of the devices in the LAN it serves or in other LAN's. A device within its LAN may transmit, for example on start-up, a local packet requesting identification of a router and the router will transmit a packet identifying its address for communication. If the devices wishes to communicate with a device in another LAN, the device may transmit a packet to the router a packet containing relevant LAN and device addresses and any data required to be transmitted. The router then places the packet on the primary cable medium. A similar router serving the LAN in which the other device is located will retrieve the packet from the primary cable medium in response to the contained LAN address and transfer the packet to its LAN where the other device can receive the packet.
A conventional bridge monitors transmission of packets within individual LAN's, extracts device addresses from the packets, and builds a table of such addresses. Unlike a router, it is effectively aware of the network devices located in individual LAN's. If a particular device in a LAN served by the bridge transmits a packet addressed to a device in another LAN, the bridge relies on its table to convey the packet from the source device toward the destination LAN and ultimately the addressed device. The bridge will similarly recognize a packet addressed to the particular device and conveyed in the primary cable medium and transfer the packet to the LAN in which the device is located. The process of recognizing a device and providing packet transferring services to the device is referred to herein as "registration." A device receiving such packet transferring services is referred to herein as being "registered" with such packet transferring apparatus. The registration process associated with a conventional bridge is entirely "transparent" to the network. Although a device must be registered with the bridge to receive packet transferring services, a network device never addresses the bridge nor seeks to determine the presence of a bridge before attempting to transmit packets to a device in another LAN.
It is sometimes desirable to couple a portable device to such a network without requiring wiring to a router or to the primary cable medium. For example, it might be advantageous to permit a personal computer or terminal to be operated anywhere in an office complex to access data from a mainframe computer. In an automated system for identifying grocery prices, a battery-operated hand-held unit might be used to scan bar-codes identifying a particular product on display shelves. The scanned data can then be transmitted to a central computer that returns the price currently recorded and applied to purchase of the product at cash registers, for comparison with displayed prices. A battery-operated hand-held unit may also be used during physical inspection of inventories, allowing an individual to identify types and quantities of products and to transmit such data to a central computer that updates inventory records.
Present practices respecting coupling of portable devices generally involve multiple spaced-apart transceivers. Broadcasting regulations often restrict the transmission power of any single source. General concern regarding possible harmful effects of excessive electromagnetic radiation may itself necessitate limited transmission power. Transceiving devices may consequently be connected to the primary cable medium of the network at various spaced-apart locations, each serving portable devices in a particular limited area. Repeaters may be used to relay packets through air where direct connection of a transceiver to the primary cable medium is inconvenient. The various areas served by the transceiving devices and repeaters will be overlapped to ensure continuous communication with mobile devices throughout an office or industrial complex.
Several shortcoming exist in such conventional practices. When a packet addressed to a particular portable device is transmitted along the cable medium, each transceiving device of the network transmits the packet through air, regardless whether the portable device is within its area. If two or more transceiving devices detect a packet from a portable unit, each causes a separate copy of the packet to be conveyed along the primary cable medium. Conventional packet handling routines provide methods for eliminating duplicate packets. However, such routines require some processing time, and multiplication of packets, particularly in a large network, can have deleterious effects on the overall rate at which data is transmitted throughout the network. The problem is aggravated by conventional error checking practices that result in re-transmission of packets from an original source when transmission errors arise. Radio transmission of packets is generally subject to higher error rates than conventional transmission over cables.
Another shortcoming in conventional practices relates to operation of battery-powered portable units. Transmission or receipt of packets can place considerable current demand on a small battery and can severely shortens its useful life. It is now common to shut off the transmitter of a battery-powered unit between its transmissions. The receiver must, however, remain operative as conventional network practice assumes that a device will be continuously "listening" for packets addressed to it. With typical current drain of several hundred milliamperes, two standard AA batteries might be expected to have a life in the order of minutes. One proposed arrangement is to discontinue all network communications with a battery-operated device until certain keys associated with the device are operated. That may be effective where the device initiates all communications between itself and other network devices, but it is not a realistic solution for a large-scale network where multiple devices in remote LAN's may wish at any time to transmit packets to the portable device.
The present invention in its different aspects provides novel communication controllers and novel methods of placing portable devices in communication with networks and addresses various shortcomings associated with prior practices.